Tandem image-forming devices are well known in the art. This type of image-forming device is typically provided with a photosensitive member for each of the colors yellow, magenta, cyan, and black, for example. The photosensitive members are juxtaposed along the circulating direction of a paper-conveying belt. Color images carried on the photosensitive members are thus transferred onto paper conveyed on the belt.
However, if the positions at which the color images are formed on the paper deviate from each other in this tandem image-forming device, the resulting color image is not registered properly. Hence, aligning the formation positions of the color images is vital.
To this end, Japanese unexamined patent application publication No. HEI-11-327249 discloses an image-forming device for detecting offset in the formation positions of the color images and for calibrating these positions. More specifically, this image-forming device forms a registration pattern configured of yellow, magenta, cyan, and black patterns on the conveying belt, each color pattern including a plurality of marks arranged along the conveying direction of the belt. The positions of marks constituting the color patterns formed on the belt vary according to positional offset of the corresponding colored images. Therefore, the image-forming device sets one of the colors yellow, magenta, cyan, or black as a reference color, measures distances between marks in the pattern of the reference color and the patterns of the other colors based on detection signals outputted from photosensors detecting the positions of the marks, and determines whether these distances match predetermined values. If the distances do not match, then the image-forming device determines that the color images are out of registration and performs calibration to correct this registration error.